High-speed mercury diffusion pump



G. R; STOLTENBERG HIGH-SPEED MERCURY DIFFUSION PuuP March 7, 1950 2 Sheets-#Sheet l Filed Sept. 18, 1947 FlELl.

w e na m WM, W n e Patented jrs HGH-SPEED MERCURY DKFFUSN x` Application September 18, i947, Serial No. 7141977@ The present invention relates to vacuum pumps and in particular it pertains to improvements in small sized mercury vapor diffusion pumps.

"Didusion pumps of the type illustrated in the present case have a Wide commercial utility for evacuating containers, such as electric discharge Claims. (Ci. E30-101) tubes and the like, to pressures of the order of a small fraction of a micron. Basically, such pumps make use of a stream of vapor nowing through a small orice from the boiler, the orice being so positioned that the vapor stream entrains gas molecules ilowing from a container in which the vacuum is to be produced. The Vapor stream drives these molecules in the direction of a piping system that flows eventually into the atmosphere. The vapor itself is condensed on la cold surface in the path of the outflowing of the gases, and is returned to the boiler for reuse. It is customary to provide a backing pump of the mechanical type at the outlet from the diffusion pump proper. The purpose of the backing pump is to reduce the pressure at the diffusion pump outlet to the equivalent of 50 microns of mercury or less.

The object of the present invention is to increase the pumping speed of a conventional mercury type diffusion pump. `A further object of this invention is to provide a small diffusion j pump capable of producing a low ultimate pressure and of such a design as to permit an increase in the exhaust or forepressure of the pump.

The foregoing objects are obtained by employing pump chimneys having such eiective crosssectional areas as to deliver the mercury vapor to the several pumping jets in a predetermined ratio.

Control of mercury vapor is further obtained by extending the pump chimneys a predetermined distance into the next larger chimney so as to create an impedance path for the mercury vapor. This is accomplished without extending the inner chimneys to the bottom of the pump boiler.

The principles ofthis invention will be pointed out in more detail in connection with the illustrative embodiments of the invention appearing in the drawings, in which:

Figure 1 is a cross-sectional view of a threestage down jet, mercury pump constructed in accordance with the present invention.

Figure 2 is a cross-section of a two-stage down jet mercury diffusion pump constructed according to the present invention.

With reference to Figure 1, the main body of the pump consists of a cylindrical casing 6 closed at its lower end 2 to provide a reservoir for mercury. The mercury is vaporized by means of a suitable heater, such as an electric heater (not illustrated). The upper end of the casing i is provided with a flange 3 which can be bolted in vacuum-tight relationship to a vessel that is to be evacuated. Beads l support the lower conically flanged portion 5 of chimney t which forms a part of the third and nal pumping stage. A conical hood or umbrella l cooperates with the upper edge 8 of chimney 6 to form the third stage jet of the pump. Suitable spacers, such. as round wire, are used to maintain the hood i and edge 8 in spaced apart relationship. Cap screws 9 center the second stage chimney lli within chimney (i.

The second or middle chimney i@ is secured to hood l as by the welding at li. This chimney extends into the lower chimney 6 for a short distance below the second jet. The upper end oi chimney l0 is flared outwardly at i2 and cooperates with hood i3 to form the second jet of the pump. The inner chimney i4 is secured to hood I3 and extends inside the middle chimney a short distance below the second jet. The upper portion K of the inner chimney cooperates with cap I6 to form the first pump jet.

The outer wall of the cylinder i is cooled by means of a water jacket il, and an exhaust tube i8 is provided for exhausting gases from the pump. A baffle ring I9 surrounds chimney 6 and deilects returning mercury to the cooled side walls of casing I. The battle also prevents any vapor from back streaming from the boiler.

In the case of three-stage pumps of the type illustrated in Fig. 1, it has been determined that optimum results are obtained by a vapor ratio of approximately 1 to 3 as between the first ano second jets, and by a vapor ratio of approximately 1 to 1.5 as between the second and third pumping jets. Best results are obtained by extending the inner chimney into the middle chimney by a distance equivalent to approximately 25% to 40% of its length and by extending the middle chimney into the outer chimney by a distance equivalent to approximately the same proportion of the length of the inner chimney. The bottom porl tions of the inner and middle chimneys thus form 3 the diameter o! the outer chimneyis about 1% that of the middle chimney.

With reference to the embodiment of Fig. 1, the lengths of the chimneys are in the ratio of approximately 4:3.5 :5.8 for the inner, middle and outer chimneys, respectively. The vertical spacing of the jets is inthe ratio of 2.5:2.75 measured from the lower rim of the iet hood or umbrella, for the distance between the ilrst and second Jets and for the distance between second and third jets respectively. As a result of this relationship between the chimney length and the jet spacing, each of the two smaller chimneys extends inside the next larger chimney for a short distance below the jet associated with the said larger chimney.

Continuing with the same units of measurements, the diameters of the chimneys are in a ratio of approximately .5:1:1.63 for the inner, middle and outer chimneys, respectively, the latter chimney being further expanded by flange 5 to approximately 2.6. Upon the same basis of measurement, the pump cylinder is approximately 2.75 units in inside diameter.

By the foregoing arrangement, the cross-sectional area of the inner chimney The corresponding effective pumping area of the middle chimney is .785-.196 or .589 square unit, giving an approximate vapor ratio of l to 3 as between the rst and second pumping stages. As between the middle and outer stages, the pump vapor is divided approximately in the ratio of 1 to 1.63. This may be seen by comparing the crosssectional area of the middle chimney, .785 with the outer chimney which in this case equals 2.07-.785 or 1.28 square units.

The pump illustrated in Fig. 2 is of generally similar construction and shows a two-stage down jet mercury pump constructed in accordance with the present invention. In this pump, the chimney 20 forming part of the iirst pumping stage extends a short distance into chimney 2i below the second stage Jet.

With two-stage pumps, as with three-stage pumps, it is desirable to divide the pumping vapor available for the rst and second stage jets in a ratio of approximately 1 to 3 by making the diameter of the inner chimney approximately onehalf that of the chimney for the second stage jet. Here again. the inner chimney should be extended a distance equivalent of approximately 25% to 40% of its length into the next larger chimney in order to create an impedance path for and to direct vapors to their respective jets. However, a reasonable variation in the given ratios is permissible, and in the example shown in Fig. 2, the diameter of the inner chimney is in approximately the ratio of 1% to 1% for the outer chimney. This makes the cross-sectional area of approximately .44 square unit for the inner chimney as compared with an eiective area of 2.07-.44 or 1.63 square units for the second stage jet. This represents a ratio of eiective cross-sectional area of approximately 1 to 3.7 for the iirst and second Jets and serves to divide the mercury vapor between the two stages in that proportion.

With particular referenceto the pump of Fig. 2, the length of the inner chimney 20, as compared to that of the outer chimney 2 I, is in the approximate ratio of 6 to 6.3. The jets, measured from the lower edge of hood 23 to the lower edge oi' hood Z4, are separated by a distance of approximately 4.8 based upon similar units, and the inner chimney extends into the outer chimney for a distance equal to 1.5 or approximately 25% its length. This lower portion of chimney 20 acts as an impedance path and directs mercury vapor to the second stage iet.

By the illustrated construction it is possible to construct a mercury diffusion pump having a casing diameter of approximately 2.75 inches andcapable of a pumping speed oi 49.5 liters per second. Such a pump can produce an ultimate vacuum of 6 10'I mm. of mercury and will operate against a maximum exhaust pressure of microns of mercury.

The various parts of the pumps may be constructed of stainless steel or any suitable material that will not amalgamate with the mercury used for the pumping vapor.

In accordance with the present invention each oi the smaller chimneys is extended inside the next larger chimney a distance equal to approximately 2540% of its length, to form an impedance path and to direct the pumping vapor to the desired Jet without necessitating extension of the chimneys to the bottom of the pump boiler. This is an advantage sincethe vapors issuing from the boiler will become thoroughly mixed and evenly distributed inthe lower portion of the bottom chimney. The vapors may then be distributed to the several jets in the desired proportions by controlling the cross-sectional area of the chimneys. The area of the chimneys, as previously pointed out should lbe in the approximate ratio oi l to 3 or 4 as between the first and second jets, and in the approximate ratio of 1 to 1.6 between the second and third jets, if any.

It will be understood that the foregoing description of two embodiments of this invention are illustrative only and that numerous modifications may be made therein by men skilled in the art without departing from the spirit of the invention, the scope of which is defined in the appended claims.

I claim:

1. In combination with a multi-stage, down- Jet, mercury diffusion pump having a casing, a boiler at the bottom of said casing, and a'plurality of co-axial cylindrical vapor chimneys of diering radii within said casing, the improvement that consists in extending the lower portion of each chimney -inside the upper portion of the next larger chimney for a distance equivalent to approximately 25% to 40% of the length of said iirst chimney to provide an impedance path for pumping vapor, and the further improvement that consists in making the diameter of the chimney associated with the rst jet approximately half the diameter of the chimney associated with the second jet, whereby pumping vapor is distributed to the iirst and second jets in an approximate ratio of 1 to 3.

2. In combination with a multi-stage, down- Jet, mercury diiusion pump including a cylindrical casing, a boiler at the bottom of said casing for containing mercury and generating mercury vapor, and a plurality of co-axial cylindrical vapor chimneys of diiering radii within said casing, the improvement that consists in extending the lower portion of each chimney inside the upper portion of the next larger chimney to a point intermediate the jet associated with the larger chimney and the surface of mercury in the boiler, said extension representing a distance of approximately 25% to 40% of the length of said first chimney and forming an impedance path for mercury vapor issuing from said boiler,

and the further improvement that consists in making the diameter of the chimney associated with the first stage jet' approximately half the diameter of the chimney associated with the second stage jet whereby mercury vapor is distributed to the iirst and second jets in an approximate ratio of 1 to 3.

3. In combination with a three-stage, downjet, mercury diiusion pump including a cylindrical casing, a boiler at the bottom of said casing for containing mercury and generating mercury vapor, said casing including inner, middle and outer co-axial cylindrical vapor chimneys of diiering radii each chimney being associated with a pumping `iet, the improvement that consists in extending the lower portion of the inner chimney inside the upper portion of the middle chimney to a point intermediate the jet associated with the middle chimney and the surface of mercury in the boiler, said extension representing approximately 25% to 40% of the length of the inner chimney, and extending the lower portion of the middle chimney inside the upper portion of the outer chimney to a point intermediate the jet associated with the outer chimney and the surface of mercury in the boiler, said extension representing approximately 25% to 40% of the length of the middle chimney whereby impedance paths are formed and the mercury vapor is directed to the pump jets, and the further improvement that consists in making the diameter of the inner chimney approximately half that of the middle chimney and in making the diameter of the outer chimney approximately 1.63 that of the middle chimney whereby mercury vapor is divided between the rst and second jets in an approximate ratio of 1 to 3 and whereby mercuryv vapor is divided between the third jet and second jet in an approximate ratio of 1.63 to 1.

4. In combination with a two-stage, down-jet, mercury diffusion pump including a cylindrical casing, a boiler at the bottom of said casing for containing mercury and generating mercury vapor, said casing including an inner and an outer co-axial cylindrical vapor chimney of differing radii, said inner chimney being associated with the first stage jet and said outer chimney being associated with the second stage jet, the improvement that consists in extending the lower portion of the inner chimney inside the outer chimney to a point intermediate the second jet and the surface of mercury in the boiler, said extension representing a distance equal to approximately 25% to 40% of the length of the inner chimney, whereby an impedance path is formed and mercury vapor is directed to the second jet, and the further improvement that consists in making the diameter of the inner chimney approximately half that of the outer chimney whereby mercury vapor is divided between the nrst and second jets in an approximate ratio of 1 to 3.

GLENN R. STOLTENBERG.

No references cited. 

